The role of tracheal smooth muscle contraction on neonatal tracheal mechanics.

The ability of tracheal smooth muscle tone to modulate the mechanical properties of neonatal airways was evaluated in six newborn lambs. Tracheal pressure-volume relationships, isovolumic compliance, hysteresis, and the relaxation time constant of the smooth muscle were evaluated as a function of incremental cholinergic stimulation. Tracheal active tensions were also determined at the graded levels of cholinergic stimulation. Data show that a maximal cholinergic stimulation resulted in a mean developed active tension value of 14.3 +/- 2.13 SEM X 10(3) dynes/cm. The resultant 55% decrease in tracheal compliance was linearly correlated to the increase in active tension (r = 0.90, p less than 0.01). Cholinergic stimulation also resulted in a 18.4% decrease (p less than 0.02) in the relaxation time constant of the smooth muscle, and in a 82% increase (p less than 0.01) in tracheal hysteresis. This study demonstrates that both elastic and viscoelastic properties of the neonatal lamb trachea are effected by tracheal smooth muscle contraction. Since neonatal airways are more compliant than adult airways, cholinergic stimulation could conceivably enhance airway rigidity and may provide a protective mechanism against pressure-induced deformation.